For many reasons, it is in some cases potentially advantageous to replace metal structural parts with reinforced organic polymers. Among the advantages the reinforced organic polymers offer include better resistance to corrosion, the ability to produce parts having complex geometries, and in some cases a superior strength-to-weight ratio. It is this last attribute that has led, and continues to lead, the adoption of reinforced polymers in the automotive industry as replacement for metal structural elements such as chassis members and other structural supports.
Epoxy resin systems are sometimes used as the polymer phase in such composites. Cured epoxy resins are often quite strong and stiff, and adhere well to the reinforcement. An advantage of epoxy resin systems, compared to most thermoplastic systems, is that low molecular weight, low viscosity precursors are used as starting materials. The low viscosity is an important attribute because it allows the resin system to penetrate easily between and wet out the fibers that usually form the reinforcement. This is necessary to avoid cosmetic blemishes such as flow lines and to produce a high strength composite.
Despite the potential advantages of these polymer composites, they have achieved only a small penetration into the automotive market. The main reason for this is cost. Metal parts can be produced using very inexpensive stamping processes that have the further advantage of producing parts at high operating rates. Polymer composites, on the other hand, must be produced in some sort of mold in which the polymer and reinforcing fibers are held until the polymer cures. The time required for this curing step directly affects production rates and equipment utilization, and therefore costs. Epoxy systems used for making these composites have required long in-mold residence times, and so the production cost has for the most part not been competitive with metal parts. Because of this, the use of epoxy resin composites to replace stamped metal parts has been largely limited to low production run vehicles. It is believed that in-mold curing times need to be reduced into the range of approximately 1 to 3 minutes for epoxy composites to become competitive with stamped metal parts for high production volume vehicles.
There are many known processes to make the fiber-reinforced composites. One of such processes is a so called resin-transfer process. One example is resin transfer molding. In this process, the reinforcing fibers are formed into a preform which is placed into a mold. A mixture of an epoxy resin component and a hardener is then injected into the mold, where it flows around and between the fibers, fills the cavity and cures to form the composite. It is advantageous that an internal mold release agent is included in the resin system. With such internal mold release agent, the resin system become less sticky and the composite can be released from the molds more easily.
The mold-filling step of these processes often takes 15 to 60 seconds or even more, depending on the size of the part and the particular equipment being used. During the entire mold-filling process, the resin system must maintain a viscosity low enough to allow it to flow between the reinforcing fibers and completely fill the mold. Resin systems formulated to cure rapidly also tend to build viscosity quite rapidly. If the fibers are pre-heated, which is often the case, the resin system can react very rapidly at points of contact with the heated fibers. The viscosity increase that accompanies this premature curing makes it difficult for the epoxy resin system to penetrate between fibers and wet them out. This results in moldings having problems that range from cosmetic (visible flow lines, for example) to structural (the presence of voids and/or poor adhesion of the cured resin to the reinforcing fibers, each of which leads to a loss in physical properties).
The problem of too-rapid viscosity build usually cannot be overcome by increasing operating pressures (i.e., the force used to introduce the resin system into the mold) because doing so can move the reinforcing fibers around within the mold, leading to spots that have little or no reinforcement and other regions in which the fibers are packed more densely. This causes inconsistent properties throughout the part, and a general weakening of the composite as a whole. Therefore, an epoxy resin system useful in resin transfer molding (and related) processes should not only have a low initial viscosity and cure rapidly, but should also build viscosity slowly during the initial stages of cure.
Another important consideration is the glass transition temperature of the cured resin. For curing epoxy resin systems, the glass transition temperature increases as the polymerization reactions proceed. It is generally desirable for the resin to develop a glass transition temperature in excess of the mold temperature so the part can be demolded without damage. In some cases, the polymer must in addition achieve a glass transition temperature high enough for the part to perform properly in its intended use. Therefore, in addition to the curing attributes already described, the epoxy system must be one which can attain the necessary glass transition temperature upon full cure.
A glass transition temperature greater than 100° C. is generally regarded as a minimum requirement for many structural composites; a preferred glass transition temperature is 110° C. and a more preferred glass transition temperature is 120° C. or more. This glass transition temperature ideally develops while the part is on the mold, rather than in some post-curing process, so that the composite is strong and rigid upon demolding and so can be demolded without being damaged, and additional costs of performing a post-curing step can be avoided.
The glass transition temperature of known resin systems can be increased through the addition of a cycloaliphatic diamine crosslinker such as isophorone diamine. However the cycloaliphatic diamine reacts more slowly, and as a result it is necessary to increase mold temperatures very significantly in order to obtain short demold times. Even at a 120° C. mold temperature, demold times can be 50 to 100% longer when the cycloaliphatic diamine crosslinker is present. If higher mold temperatures are used, the open time becomes too short. Therefore, this system provides enhanced glass transition temperature at the expense of a much longer demold time and/or a much shorter open time, depending on the mold temperature that is selected. In any event, much higher mold temperatures are needed than is the case when the cycloaliphatic diamine is omitted.
Another very significant issue with the foregoing is the presence of diethylene triamine, which is coming under regulatory pressure in some jurisdictions. There is a strong desire to replace diethylene triamine with an alternative hardener, while retaining the benefits of low initial viscosity, good open time and fast cure. A higher glass transition temperature would be a further advantage, if it could be obtained without comprising the needed curing characteristics.
WO2014078218, incorporated herein by reference in its entirety, provides a novel method for producing good quality fiber-reinforced epoxy resin composites with short cycle times. The epoxy resin system has a long open time and a low initial viscosity, and cures rapidly to produce a composite in which the resin phase has a glass transition temperature of at least 110° C. and preferably at least 120° C. Applicants in WO2014078218, incorporated herein by reference in its entirety, found that the combination of epoxy resin, a polyethylene tetraamine mixture hardener and triethylene diamine provides a unique and unexpected combination of extended open time and fast cure, while at the same time producing a high (>110° C.) glass transition temperature cured polymer. Mold temperatures needed to accomplish this generally do not exceed 130° C.
However, it was also learned that in order to advance to use of carbon fiber composites in vehicles, it is important that these composites should be visually appealing in areas of transportation that might be more visible. As such, the coloring (black or shade thereof) of resin systems used in the manufacture of carbon fiber composites can be useful in order to improve the aesthetics of the final composites. It is desired to use these color improvement approaches to cover effects caused by yellowing of the aromatic based epoxy resin systems, cover up blemishes caused during production, and color the white polyester threads used to stitch fabric performs together in order to make them less visible. Finding a dye that is stable in the resin systems due to their often reactive nature and in the higher temperature conditions used during manufacture of the composite can be a challenge. The dye should be non toxic but also highly soluble and stable in the resin systems. Furthermore, the dye must not impact the reactivity or performance either during the process or of the final part produced and the resin system should still be able to manufacture composites in fast cycle times of several minutes or less.